[buzz bomb]

where did you tap the speaker wires for the amp? also did you check the wiring routes you used to make sure the factory harness did not get pinched? the can bus is very sensitive and does not like to be molested/pinched/cut/pierced/

 

[buzz bomb]

so by going to the left b post you should have been only to grab the left rear speaker. that is not a great way/area to pick off feeds to the amp just due to space in that spot. i will always run back to the head unit for any kind of tap and will not use taps of any kind i just don't trust them. the only other thing i can think of is that the vehicle ground is not large enough to handle the amperage. you may want to run a heavy ground with your fused power feed back to the amps.